Thermochromic materials undergo marked changes in reflectance with changes in temperature. Thin films of thermochromic materials exhibit hysteresis with temperature changes, which makes them especially useful for recording and storing images.
Images can be created in these films at extremely high speeds by heating a region of the film with a laser or electron beam. As long as the ambient temperature of the film is maintained at a suitable recording temperature inside the hysteresis loop, these images will be stored in the film. Normally, these images are erased by allowing the entire film to cool from its recording temperature to some temperature below the loop. This radiative cooling, however, is extremely slow and impractical in applications calling for rapid writing and erasing of images. More direct means have been employed for cooling these films, such as direct cooling with thermoelectric junctions. These junctions can cool small areas of film relatively quickly, but are impractical for erasing large image displays because of the number of junctions required to cover the entire surface area of the display. Accordingly, there is a need for a method and apparatus for erasing thermochromic films at high speeds.
Certain pressure sensitive materials are known which exhibit properties similar to thermochromic films except that they undergo local changes in reflectance upon application of pressure or strain. Although these materials offer potential for displaying high resolution images, it has heretofore been difficult to exploit the optical properties of these materials because images cannot be "written" in the film by an electron beam or a laser. These materials, however, offer many advantages for "writing" and erasing images. A need thus arises for an apparatus and method for creating and erasing images in pressure sensitive materials.